Conventionally, active noise control (ANC) conventionally is known as a technique that suppresses periodic noise such as the operating sound of a motor or engine. The ANC technique generates a signal (control sound) having the same amplitude as that of noise and a phase opposite to that of the noise, and reduces the noise by sound wave interference. The ANC technique is used, for example, to reduce the noise in car cabins and reduce the effect of environmental noise when using headphones.
As a method of generating the control sound, a method that applies an adaptive notch filter to a sine wave and cosine wave output from a fundamental sound source and synthesizes a signal after adaptation is known. FIG. 1 is a view showing an example of the arrangement of an active noise suppressor using the adaptive notch filter.
This active noise suppressor comprises an adaptive notch filter 100, a cosine wave generator 121 and sine wave generator 122 forming a fundamental sound source, transfer elements 101 and 102 that respectively apply transfer functions C0 and C1 of a premeasured system to the output frequency of the fundamental sound source, an adder 103 that adds the outputs from the transfer elements 101 and 102 and outputs the sum as a reference signal r, and an adaptive control algorithm calculator (filter coefficient calculator) 110.
The cosine wave generator 121 and sine wave generator 122 respectively output a cosine wave signal and sine wave signal having a frequency equal to a peak frequency f of premeasured noise, and having a predetermined amplitude. These fundamental signals are supplied to the transfer elements 101 and 102 that respectively apply the transfer coefficients C0 and C1 premeasured for a signal having the frequency f, and to the adaptive notch filter 100.
The adaptive notch filter 100 multiplies the cosine wave and sine wave signals by filter coefficients W0 and W1, respectively, supplied from the adaptive control algorithm calculator 110, and outputs the signals. An adder 130 adds the output signals from the adaptive notch filter 100, and the obtained signal is output as a control sound from, for example, a loudspeaker (not shown).
The adaptive algorithm calculator 110 receives an error signal e (a difference between the control signal and target noise) obtained by a microphone 140 and the reference signal r output from the adder 103, and calculates and updates the coefficients W0 and W1 of the notch filter 100 by an adaptive algorithm such as an LMS (Least Mean Square) algorithm so as to reduce the error signal e.
Patent Reference 1: Japanese Patent Laid-Open No. 11-325168